Unless I'm mistaken, it looks like you're using one giant sprite to constitute the cloud you're trying to make.

Not being a particle system expert, I'd recommend using multiple sprites with random size, slightly random color (light gray/dark gray), and some subtle rotation as well. Something more along these lines, I think.

Try using ps_area_fog_xlarge and/or ps_fog_sky. The area fog looks good as ground fog but putting some up higher in the map could pass for some cloud cover. The sky fog is a flat moving level of fog. It's only visible from the bottom though so don't panic if you can't see it from the top, just look at it from below

On another note: Fog areas are more of a global thing, they look pretty awful if you're looking outside-in. Don't try to use them for clouds

I use cgtextures.com for skyboxes. My free account lets me use whatever images I want at a limited size, which is often more than enough for a skybox. In a worst-case scenario I do some editing in GIMP and make it passable.